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International audienceLow-angle normal faults accommodate a large part of continental post-orogenic extension. Besides the intrinsic rheological characteristics of the continental crust that may lead to the formation of shallow-dipping shear zones at the brittle–ductile transition, the role of pre-existing low-angle structures such as large thrusts has been proposed by several authors. We explore this question with the example of the North Cycladic Detachment System (NCDS) that is composed of a series of distinct detachments cropping out on the islands of Andros, Tinos and Mykonos, separating the Cycladic Blueschists in the footwall from the Upper Cycladic Nappe in the hanging wall. We show that these extensional structures are part of a single large-scale structure (more than 200 km along strike) that reactivates the Vardar suture zone. It extends eastward on Ikaria and westward offshore Evia and Thessalia where it probably connects to recent shallow-dipping normal faults evidenced on published seismic reflection profiles. The NCDS started its activity in the Oligocene concommitantly with the Aegean extension, and was still active in the Late Miocene. It has exhumed a series of metamorphic domes from southern Evia to Mykonos below low-angle detachment systems, made of low-angle normal faults and low-angle ductile shear zones. The ductile shear zones and the faults were created with a low dip and they kept the same attitude throughout their exhumation. We identify three main detachments that are part of a continuum of extension on the NCDS : Tinos detachment, Livada detachment and Mykonos detachment. A fourth detachment (Vari detachment) is the reactivation of an Eocene exhumation-related structure. Deformation in the footwall is characterized by intense stretching and flattening. Using the spatial evolution observed along strike from Andros to Mykonos we construct a history of formation of the NCDS starting with the reactivation of former thrusts leading to the exhumation of high-temperature metamorphic domes. The Aegean example shows that reactivation of earlier shallow-dipping discontinuities can play a fundamental role in continental post-orogenic extension

[1] The dynamic processes leading to synconvergent exhumation of high-pressure lowtemperature (HP-LT) rocks at oceanic accretionary margins, as well as the mechanisms maintaining nearly steady state regime in most accretion prisms, remain poorly understood. The present study aims at getting better constraints on the rheology, thermal conductivity, and chemical properties of the sediments in subduction zones. To reach that goal, oceanic subduction is modeled using a forward visco-elasto-plastic thermomechanical code (PARA(O)VOZ-FLAC algorithm), and synthetic pressuretemperature-time (P-T-t) paths, predicted from numerical experiments, are compared with natural P-T-t paths. The study is focused on the well constrained Schistes Lustrés complex (SL: western Alps) which is thought to represent the fossil accretionary wedge of the Liguro-Piemontese Ocean. For convergence rates comparable to Alpine subduction rates ($3 cm yr À1 ), the best-fitting results are obtained for high-viscosity, low-density wedge sediments and/or a strong lower continental crust. After a transition period of 3-5 Ma the modeled accretionary wedges reach a steady state which lasts over 20 Ma. Over that time span a significant proportion ($35%) of sediments entering the wedge undergoes P-T conditions typical of the SL complex ($15-20 kbar; 350-450°C) with similar P-T loops. Computed exhumation rates (<6 mm yr À1 ) are in agreement with observations (1-5 mm yr À1 ). In presence of a serpentinite layer below the oceanic crust, exhumation of oceanic material takes place at rates approaching 3 mm yr À1. In all experiments the total pressure in the accretionary wedge never deviated by more than ±10% from the lithostatic component.Citation: Yamato, P., P. Agard, E. Burov, L. Le Pourhiet, L. Jolivet, and C. Tiberi (2007), Burial and exhumation in a subduction wedge: Mutual constraints from thermomechanical modeling and natural P-T-t data (Schistes Lustrés, western Alps),

International audience19 The extent to which heat recorded in orogens reflects thermal conditions inherited from 20 previous rift-related processes is still debated and poorly documented. As a case study, we 21 examine the Mauléon basin in the north-western Pyrenees that experienced both extreme 22 crustal thinning and tectonic inversion within a period of ~30 Myrs. To constrain the time-23 temperature history of the basin in such a scenario, we provide new detrital zircon fission-24 track and (U-Th-Sm)/He thermochronology data. The role of rift-related processes in 25 subsequent collision is captured by inverse modeling of our thermochronological data, using 26 relationships between zircon (U-Th-Sm)/He ages and uranium content, combined with 27 thermo-kinematic models of a rift-orogen cycle. We show that the basin recorded significant 28 heating at about 100 Ma characterized by high geothermal gradients (~80°C/km). Our 29 thermo-kinematic modeling and geological constraints support the view that subcontinental 30 lithospheric mantle was exhumed at that time below the Mauléon basin. Such a high 31 geothermal gradient lasted 30 Myr after onset of convergence at ~83 Ma and was relaxed 32 during the collision phase from ~50 Ma. This study suggests that heat needed for ductile 33 shortening during convergence, is primarily inherited from extension rather than being only 34 related to tectonic and/or sedimentary burial. This should have strong implications on tectonic 35 reconstructions in many collision belts that resulted from inversion of hyper-extended rift 36 basins

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International audienceIn this paper we describe a computational methodology that is specifically designed for studying three-dimensional geodynamic processes governed by heterogeneous visco-plastic Stokes flow. The method employs a hybrid spatial discretization consisting of a View the MathML sourceQ2-P1disc mixed finite element formulation for the Stokes problem, coupled to a material-point formulation which is used for representing material state and history-dependent variables. The applicability and practicality of this methodology is realized through the development of an efficient, scalable and robust variable viscosity Stokes preconditioner. In this work, these objectives are achieved through exploiting matrix-free operators and a geometric multigrid preconditioner employing hybrid coarse level operators, Chebyshev smoothers and hybrid Krylov coarse level solvers. The robustness and parallel efficiency of this strategy is demonstrated using an idealized geodynamic model. Lastly, we apply the new methodology to study geodynamic models of continental rifting and break-up in order to understand the diverse range of passive continental margins we observe on Earth today

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